Urea is generally produced from ammonia and carbon dioxide. It can be prepared by introducing an ammonia excess together with carbon dioxide at a pressure between 12 and 40 MPa and at a temperature between 150° C. and 250° C. into a urea synthesis zone. The resulting urea formation can be presented best in the form of two consecutive reaction steps, in the first step ammonium carbamate being formed according to the exothermic reaction:2NH3+CO2→H2N—CO—ONH4 after which the ammonium carbamate formed is dehydrated in the second step to give urea according to the endothermic equilibrium reaction:H2N—CO—ONH4↔H2N—CO—NH2+H2O
The extent to which these reactions take place depends among other things on the temperature and the ammonia excess used. The reaction product obtained in a urea synthesis solution substantially consists of urea, water, unbound ammonia and ammonium carbamate. The ammonium carbamate and the ammonia are removed from the solution and are generally returned to the urea synthesis zone.
In addition to the above-mentioned solution in the urea synthesis zone, a gas mixture is formed which consists of unconverted ammonia and carbon dioxide together with inert gases, the so called reactor off-gas. The urea synthesis section may comprise separate zones for the formation of ammonium carbamate and urea. These zones may also be combined in a single apparatus.
The invention pertains to a process for the preparation of urea according to a stripping process, as conducted in a urea stripping plant. Such a process is described in, inter alia, Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350.
In a urea stripping plant the decomposition of the ammonium carbamate that has not been converted into urea and the expulsion of the usual ammonia excess largely takes place at a pressure that is essentially almost equal to the pressure in the synthesis reactor. This decomposition and expulsion take place in one or more stripper(s) installed downstream of the reactor, possibly with the aid of a stripping gas such as, for example, carbon dioxide and/or ammonia, and with the addition of heat. It is also possible to apply thermal stripping. Thermal stripping means that use is made exclusively of the supply of heat to decompose ammonium carbamate and remove the ammonia and carbon dioxide present from the urea solution. The gas stream leaving a stripper contains ammonia and carbon dioxide which are condensed in a high-pressure condenser and then returned to the urea synthesis zone.
In a urea stripping plant the synthesis zone is operated at a temperature of 160-240° C. and preferably at a temperature of 170-220° C. The pressure in the synthesis reactor is 12-21 MPa, preferably 12.5-20 MPa, more preferably 13-16 MPa. In the art, these ranges are generally considered to represent “high pressure” (as also used in connection with a conventional “High Pressure Carbamate Condenser”). The gross ammonia to carbon dioxide molar ratio (gross N/C ratio) in the urea synthesis zone of a stripping plant usually is in between 2.2 and 5 and preferably between 2.5 and 4.5 mol/mol. For completeness' sake, it is noted that the synthesis zone will usually operate on the basis of both an external feed of the starting materials, ammonia and carbon dioxide, and recycled starting materials, generally comprising recycled ammonia and carbon dioxide in a free form as well as in the form of ammonium carbamate and/or biuret. The gross N/C ratio, which is a term having a customary meaning in the art, refers to a hypothetical mixture in which all starting materials are converted into free ammonia and carbon dioxide.
The synthesis zone can comprise a single reactor or a plurality of reactors, arranged in parallel or in series. In addition to one or more reactors, the synthesis section comprises a stripper, a condenser and a scrubber, all operating at substantially the same pressure. The synthesis zone is generally referred to as a High Pressure (HP) section.
In the synthesis section the urea solution leaving the urea reactor is fed to a stripper in which a large amount of non-converted ammonia and carbon dioxide is separated from the aqueous urea solution. Such a stripper can be a shell and tube heat exchanger in which the urea solution is fed to the top part at the tube side and a carbon dioxide feed to the synthesis is added to the bottom part of the stripper. At the shell side, high pressure (HP) steam is added to heat the solution via indirect heat exchange. The urea solution leaves the heat exchanger at the bottom part, while the vapor phase leaves the stripper at the top part. The vapor leaving said stripper contains ammonia, carbon dioxide and a small amount of water. Said vapor is condensed in a falling film type heat exchanger or a submerged type of condenser that can be a horizontal type or a vertical type. A horizontal type submerged heat exchanger is described in the aforementioned Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350.
After the stripping treatment, the pressure of the stripped urea solution is reduced in a urea recovery section. In the recovery section the non-converted ammonia and carbon dioxide in the urea solution are separated from the urea and water solution. A recovery section comprises usually a heater, a liquid/gas separation section and a condenser. The urea solution entering a recovery section is heated to vaporize the volatile components ammonia and carbon dioxide as well as water from that solution. The heating agent used in the heater is usually steam. The ammonium carbamate aqueous solution formed in a low pressure carbamate condenser in the recovery section, operated at a lower pressure than the pressure in the synthesis section, is preferably returned to the urea synthesis section operating at synthesis pressure. The recovery section is generally a single section or can be a plurality of recovery sections arranged in parallel or in series. The recovery section comprises a heater, a liquid/gas separator and a condenser. The pressure in this recovery section is generally between 200 to 600 kPa. This section is generally referred to as a low pressure (LP) recovery section (or recirculation section, the terms “recovery section” and “recirculation section” in this description are used interchangeably). In the heater of the recovery section the bulk of ammonia and carbon dioxide is separated from the urea and water phase by heating the urea solution. Usually low pressure (LP) steam is used as heating agent. The urea and water phase contains a small amount of dissolved ammonia and carbon dioxide that leaves the recovery section and is sent to a downstream urea processing section where the urea solution is concentrated by evaporating the water from said solution. This section is typically referred to as the evaporation section and it is typically comprised of one or two evaporators, whose vapors are condensed downstream and recycled back to the process.
In some embodiments, in addition to the HP synthesis section and the LP recovery section, a medium pressure (MP) treatment section is present. E.g., WO 02/090323 discloses a urea process and plant of the carbon dioxide stripping type, wherein a MP treatment section is present parallel with the HP stripping section. A similar disclosure is found in EP 2 086 928.
Processes also exist in which a MP treatment section is present in series, downstream of the urea synthesis section. In this respect reference can be made to, e.g., GB 1 542 371, and other disclosures of the Snamprogetti Ammonia and Self-Stripping processes.
In the development of the field of urea production processes and plants, it is permanently sought to improve one or more of various output parameters. Thus, it is desired to further improve yield. It is also desired to improve the efficiency of recirculation of residual reactants. Further, it is desired to reduce, or at least further optimize, one or more of the energy consumption, the operational expenses (OPEX), and the capital expenses (CAPEX).
The inventors have, inter alia, identified a general challenge in respect of the energy used in a urea plant. In the plants to date, a large amount of internal energy is wasted in the expansion of the process stream due to the inevitable pressure drop between the HP synthesis section and the LP (or MP and LP) downstream sections. It would be desired to provide a urea process and plant allowing making use of at least part of said otherwise wasted internal energy. In this respect it is noted that in thermodynamics, the internal energy of a system is energy contained within the system, excluding the kinetic energy of motion of the system as a whole and the potential energy of the system as a whole due to external force fields. It keeps account of the gains and losses of energy of the system that are due to changes in its internal state. The invention refers to the use of internal energy as work (i.e., not as heat).
Background art includes EP 1491526. Therein, in a urea production process, an expansion step is provided, in a turbine, of the reaction mixture coming from high pressure synthesis. Kinetic energy produced by the turbine is put to use elsewhere in the process.
Another background reference is WO 2013/104638. This relates to the use of a passivating agent in an otherwise conventional thermal stripping process for producing urea. As is typical for thermal (self) stripping processes, the stripper is operated at a pressure slightly below that of the urea synthesis reactor.